1. Field of the Invention
This invention relates to flexible graphite and articles of flexible graphite.
2. Background
Graphite sheets are generally made from expanded vermicular particles which are compressed to form graphite-based sheets. One typical way to make expanded vermicular particles is to treat flakes of natural graphite with an acid solution then expose the treated flakes to high temperature. This causes the graphite flakes to expand in a direction perpendicular to the crystal plane of graphite atoms (typically referred to in the industry as the xe2x80x9ccxe2x80x9d direction in crystallographic terminology). A form of a substantially flat, flexible, integrated graphite sheet is made by compressing expanded graphite particles which are at least 80 times that of the original particles under a predetermined load and in the absence of a binder. Each particle can be held together without a bonding element. The density and thickness of the sheet may be varied by controlling the degree of compression.
A graphite sheet made by the process described above shows suitable properties to be used for gasket and seal applications. The normal requirements for gasket and seal applications include compressibility, recovery, resistance to chemical attack, high yield point and low creep value that collectively produce high sealability of gasket material that results in the gasket""s ability to inhibit leakage of gas or liquid through a seal. Creep describes the progressive deformation of a material at constant stress. A creep value is measured as a strain after a certain time of applying compressive stress on a material. The yield point or crush point of a material (e.g., gasket material) describes that the highest compressive stress point at which the material will no longer decrease in thickness without also extruding in planar dimensions.
Satisfying the requirements of compressibility, recovery, resistance to chemical attack, high yield point and low creep concurrently to the degree required by commercial applications is hard to accomplish and the properties of the graphite sheet also depend upon the environment in which it is utilized and density and thickness of the products, though graphite exhibits suitable over all properties as a gasket. It is particularly difficult to combine low creep and high conformability. Elastic materials that conform well to rough surfaces tend to creep under high pressure.
Graphite material generally has good chemical resistance. Graphite material, however, also has a high affinity to wetting agents, such as oils. Such affinity contributes to erosion of graphite-based gaskets, for example, in internal combustion engines. Furthermore, the high temperature of this environment makes it easier for oil to penetrate between layers of graphite. Thus, graphite material exposed to an environment where oil is abundant can exhibit a high creep value and low sealability. Graphite sheet consists of layers of carbon atoms which are held together by weak van der Waals forces. This weak force between the atomic layer of the xe2x80x9ccxe2x80x9d direction may be the source of the leakage when the graphite sheet is used as a gasket and seal because foreign substance can penetrate between these layers of material.
It is known that to reduce penetration and to increase bonds between graphite particles, the graphite material may be impregnated with certain resins.
In the case of gasket applications, however, desirable properties of compressibility and recovery limit the use of resin additives. Similarly, high temperature applications of certain gaskets also limits the range of possible resin additives. In Japanese Patent No. 53-44917, it has been suggested to use polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCFE) or PTFE-hexafluoroethylene copolymers as possible resin additives. In a dry state, it is believed that desirable particle sizes of less than 20 microns of these materials are generally not commercially available. Increasing the density of the sheet by utilizing resin particles of particle sizes of 20 microns or more improves penetration resistance. However, high resin content leads to low recovery and higher creep relaxation that limit the use of gasket material. See John H. Bickform, xe2x80x9cGaskets and Gasketed Joints,xe2x80x9d 1997, published by Marcel Dekker, Inc., New York.
An article of manufacture is disclosed. In one embodiment, the article of manufacture includes graphite, such as in an amount of about 90 to 96 percent by weight,and a fluoro-resin including ethylene-tetrafluoro ethylene (ETFE) copolymers. One example includes about 4 to 10 percent of the fluoro-resin. The graphite is intertwined by fibrils of the fluoro-resin. In one aspect, the article of manufacture is a sheet suitable for use as a gasket material. The sheet is suitable for use in the presence of wetting agents, including oil and includes properties of improved tensile strength, sealability and yield point especially in comparison after oil immersion over prior art graphite sheets (e.g., gaskets).
A method is also disclosed. In one embodiment, the method comprises mixing vermicular graphite particles with a fluoro-resin including ETFE copolymers, forming a desired article (such as a sheet) of the mixture, and heating the article to a temperature above the melting point of the ETFE copolymers.